1.2 Example 1: A simple hydraulic system

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1 Note: It is possible to use more than one fluid in the Hydraulic library. This is important because you can model combined cooling and lubrication systems of a library. The hydraulic library assumes a uniform temperature throughout the system. If thermal effects are considered to be important, the Thermal Hydraulic and Thermal Hydraulic Component Design libraries should be used. There are models of cavitation and air release in the hydraulic library. Note also there is a special two-phase flow library. A typical application for this is air conditioning systems. Chapter 1 of the manual consists of a collection of tutorial examples. We strongly recommend that you do these tutorial examples. They assume you have a basic level of experience using AMESim. As an absolute minimum you should have done the examples in Chapter 3 of the AMESim manual and the first example of Chapter 5 which describes how to do a batch run. 1.2 Example 1: A simple hydraulic system Objectives Construct a very simple hydraulic system Introduce the simplest pipe/hose submodels Interpret the results with a special reference to air release and cavitation Figure 1.1: A very simple hydraulic system Pump Motor Prime mover Tank Relief valve Rotary Load In this exercise you will construct the system shown in Figure 1.1. This is perhaps the simplest possible meaningful hydraulic system. It is built partly from components from the Hydraulic category (which are normally blue) and partly from the Mechanical category. The hydraulic part is built up from standard symbols used for hydraulic systems. 2

2 Hydraulic Library 4.2 User Manual The prime mover supplies power to the pump, which draws hydraulic fluid from a tank. This fluid is supplied under pressure to a hydraulic motor, which drives a rotary load. A relief valve opens when the pressure reaches a certain value. The output from the motor and the relief valve returns to the tank. The diagram shows three tanks but it is quite likely that a single tank is employed. There are two categories in the Hydraulic library. These have blue as the standard color. If you do not have this categories displayed, check the path list in the Options menu. The first category contains general hydraulic components. The seconds contains special valves.the hydraulic components used in the model you will build can all be found in the first of these Hydraulic categories. If you click on this category icon, you will have the dialog box shown in Figure 1.2. First look at the components available in this library. Display the title of some components by moving the pointer over the icons. Figure 1.2: The components in the first hydraulic category. Step 1: Use File New... to produce the following dialog box. 3

3 Figure 1.3: The hydraulic starter system. Select the hydraulic starter circuit libhydr.amt and then click on OK. A new system with a fluid properties icon in the top left corner of the sketch will be created. You could also have clicked on the New icon in the tool bar but if you do this you will have to add the fluid properties icon yourself. Step 2: Construct the rest of the system and assign submodels 1. Construct the system with the components as shown in Figure Save it as hydraulic1. 3. Go to Submodel mode. Notice that the drop, the prime mover, the node and the pipes are not of normal appearance because they do not have submodels associated with them. The easiest way to proceed is: 4. Click on the Premier submodel button which is situated in the horizontal menu bar. 4

4 Hydraulic Library 4.2 User Manual Figure 1.4: The line submodels. 5. Press the mouse right-button. 6. Select Show line labels in the label menu. You get something like Figure 1.4. It is possible that your system may have HL000 associated with one of the other line runs. These minor variations are dependent on the order in which you constructed the lines. They will not influence the simulation results. An important feature to notice is that a line run has a special submodel (HL000) which is not a direct connection. To emphasize this point the line run has a special appearance. Remember the submodel DIRECT does nothing at all. It is as if the ports at the end of the line were connected directly together. In contrast, HL000 computes the net flow into the pipe and uses this to determine the time derivative of pressure. If the net flow into the pipe is positive, pressure increases with time. If it is negative, it decreases with time. The pressure created by HL000 is conveyed to the relief valve inlet. The motor inlet is conveyed by the node and submodel DIRECT. Step 3: Set parameters 1. Change to Parameter mode. 2. Set the following parameters and leave the others at their default values: Submodel Title Value HL000 pipe length [m] 4 RL00 coefficient of viscous friction [Nm/(rev/ min)]

5 Figure 1.5: Setting the line submodel HL000 parameters. 3. To display the parameters of a line submodel click the left mouse button with the pointer on or near the appropriate line run. Part of the dialog box for HL000 is shown in Figure 1.5. The compressibility of the oil and the expansion of the pipe or hose with pressure are taken into account together with the pipe volume. HL000 normally requires the bulk modulus of the hydraulic fluid and pipe wall thickness together with the Young s modulus of the wall material. From these values an effective bulk modulus of the combined fluid and pipe walls can be calculated. The effective bulk modulus of a hose is normally very much less than that of a rigid steel pipe. 4. Click on the fluid icon FP04 in the sketch. A new dialog box as shown in Figure 1.6 is displayed. This shows you the properties of the hydraulic fluid. Currently they are at their default values and the absolute viscosity, bulk modulus, air/gas content and temperature are given in common units. 6

6 Hydraulic Library 4.2 User Manual Figure 1.6: Parameter for fluid properties submodel FP04. Note that the first item in the list is an enumeration integer parameter. A collection of properties of varying complexity are available but for this exercise elementary is satisfactory. 5. Click on OK. Step 4: Run a simulation 1. Go to Run mode and do a simulation run. The default values in the Run Parameters dialog box are suitable for this example. 2. Click on the Start run button. 3. Click on the pump component to produce the dialog box shown in Figure 1.7. Some variables such as a pressure have no direction associated with them. A (gauge) pressure of -0.1 bar indicates that the pressure is below atmospheric. In contrast other variables, such as flow rate, do have a direction associated with them. A flow rate of -6 L/min indicates that the flow is in the opposite direction to some agreed standard direction. 7

7 Figure 1.7: The Variable List for PU001. Note that you can use the Replay facility to give you a global picture of the results. Figure 1.8 also shows the flow rates in L/min at a time of 10 seconds. Figure 1.8: Flow rates displayed in replay. 4. To plot a variable associated with a line submodel, click on or near the corresponding line run. 5. Plot pressure at port 1 for HL000. 8

8 Figure 1.9: The pressure in the hydraulic pipe. Hydraulic Library 4.2 User Manual Notice how the pressure goes up to just over the relief valve setting (150 bar). During this time the load speeds up rapidly and actually 'over-speeds'. At this point the motor is demanding more hydraulic flow than the pump can supply. The result is that the pressure must drop and the relief valve closes. The pressure continues to drop and falls below zero bar gauge. However, pressure is not like voltage or force. We cannot have a pressure of -100 bar. The absolute zero of pressure is about bar gauge. It is time to introduce two terms. Cavitation and air release When pressure falls to very low levels, two things can happen: Air previously dissolved in the fluid begins to form air bubbles. The pressure reaches the saturated vapor pressure of the liquid and bubbles of vapor appear. These phenomena are known as air release and cavitation respectively. They can cause serious damage. Using the Zoom facility, the graph gives a better view of the lower pressure values: Figure 1.10: Low pressure in the hydraulic pipe. 9

9 All AMESim submodels have hydraulic pressure in bar gauge. The low pressure shown in Figure 1.10 : Low pressure in the hydraulic pipe. is caused by the load speed exceeding its steady state or equilibrium value and it is a highly undesirable behavior as it can result in damage to the real system. In reality the starting values we have given for the pipe pressure and load speed are not very realistic and the prime mover would start from rest or some valve would be used to regulate the flow to the motor. However, hydrostatic transmission systems like this often do suffer badly from cavitation and air release problems. Note that all AMESim submodels have hydraulic volumetric flow rate in L/min. There are two possible interpretations of this flow rate: The flow rate is measured at the local current hydraulic pressure, or The flow rate is measured at a reference pressure. AMESim adopts the second alternative with a reference pressure of 0 bar gauge. This means that the volumetric flow rate is always directly proportional to the mass flow rate. In most situations the difference between the two flow rates is negligible. However, there are three situations when there is a significant difference: 1. There is a very large air content; the pressure drops below the saturation pressure for air in the liquid and air bubbles are formed in the liquid. 2. The pressure drops to the level of the saturated vapor pressure of the liquid and cavities of vapor form. 3. Extremely high variations in pressure occur such as in certain types of fuel injection systems. The first situation is called air release and the second cavitation. If there is cavitation or significant air release at the inlet to a pump, the flow rate according to the first definition will not be reduced but with the approach adopted by AMESim it will be significantly reduced. The properties of hydraulic fluids vary a great deal. Modeling them is a very specialist process and the model can be extremely simple or highly complex. The run times are greatly influenced by this level of complexity. 1.3 Example 2: Using more complex hydraulic properties Objectives: Use more complex models of fluid properties. See how air content changes the performance of the system. In the Hydraulic category two special components can modify the fluid properties: 10